The methods for preparing porous 4 group metal oxides are explained with porous titanium oxide taken as an example. The methods known today are roughly classified into two methods and one is based on combustion of titanium tetrachloride by oxygen (vapor phase method) while another is based on the preparation of hydrous titanium oxide or the precursor of titanium oxide in advance by such means as hydrolysis of titanium sulfate or titanyl sulfate, alkali neutralization of titanium tetrachloride or titanium sulfate or hydrolysis of a titanium alkoxide followed by drying and calcining the hydrous titanium oxide (liquid phase method).
Representatives of the aforementioned liquid phase methods are the following: • where the hydrolysis of titanyl sulfate is resoted to, titanyl sulfate is heated at or above 170° C. and hydrolyzed at a pressure equal to or above the prevailing saturated steam pressure to yield hydrous titanium oxide and then the hydrous titanium oxide is calcined at 400-900° C. to yield spherical anatase type titanium oxide (JP05-163,022 A); • in the case of the neutralization of titanyl sulfate, needle crystals of titanyl sulfate are contacted with an aqueous alkaline solution and the resulting needles of hydrous titanium oxide are dried and calcined to yield titanium oxide in needle crystals (JP05-139,747 A); • In case a sol-gel method involving the hydrolysis of a titanium alkoxide is adopted, a titanium tetraalkoxide and water are mixed to form a precipitate, the precipitate is filtered, washed with water, then mixed with water to form a slurry, the slurry is subjected to a hydrothermal treatment and the product thereby obtained is dried to give mesoporous titanium oxide with a pore volume of 0.1-0.5 ml/g and an average pore diameter of 3-30 nm (JP2001-031,422 A).
However, porous titanium oxide prepared by the aforementioned conventional methods generally shows extremely poor heat stability and presents the problem of suffering a rapid loss in specific surface area and failing to maintain a large surface area when subjected to calcination at high temperature or over a prolonged period of time. What happens here is that the hydroxyl groups detach themselves from hydrous titanium oxide and undergo condensation or titanium oxide being formed undergoes the so-called sintering thereby growing considerably in crystalline particles; for example, as shown in FIG. 1 which is a plot of the relationship between specific surface area and calcining temperature, crystallization or crystal transformation from amorphous to anatase and rutile takes place as the calcining temperature rises and, as a result, the specific surface area decreases rapidly which makes it difficult to maintain the specific surface area at a high level.
Under these circumstances, a catalyst carrier or a catalyst based on titanium oxide, in spite of its extremely high activity for hydrotreating per unit specific surface area, cannot maintain a large specific surface area at high temperature because of its poor heat stability nor manifest a sufficient performance as a catalyst and, unlike catalyst carriers or catalysts based on alumina or silica, has never been utilized commercially.
Where titanium oxide is intended for use as an alkylation catalyst, a high-temperature heat treatment is necessary in order to provide titanium oxide with properties of a superstrong acid; but deteriorating heat stability and diminishing specific surface area cause a decrease m the absolute amount of superstrong acid and the properties required for a catalyst could not be secured.
Where titanium oxide was intended for use as a denitrogenation catalyst of exhaust gases, it could only be used with its specific surface area limited normally to a low range of 40-50 m2/g because of its poor heat stability in spite of its excellent denitrogenation activity per unit specific surface area. Thus, the problems here were the necessity for using a large quantity of a given catalyst as well as narrowing of the range of use temperature of the catalyst caused by poor heat stability.
Moreover, where titanium oxide is used as a catalyst in the Fischer-Tropsch (FT) reaction, only titanium oxide with a small specific surface area was made available in spite of its good abrasion resistance and it has not been possible to obtain a titanium oxide-based catalyst exhibiting satisfactory performance for this particular application.
Several proposals have been made to solve the aforementioned problems. For example, attempts have been made to add a secondary component such as silica, alumina and phosphorus to titanium oxide to prepare porous titanium oxide that has a large specific surface area and shows excellent heat stability. The following are examples of such attempts.
According to a proposal made in JP07-275,701 A, a silicon compound and a titanium compound are dissolved in an acidic solution, a basic substance is then added to cause coprecipitation and the coprecipitate is aged to give silica-titanium oxide. This method regulates the ratio of titanium oxide to silica within the range of 1-50 wt % (13 wt % of titanium oxide is used in one example) and the silica-titanium oxide catalyst obtained by 3-hour calcination at 500° C. shows an extremely large specific surface area of 558 m2/g.
A proposal in JP08-257,399 A is directed to the preparation of a titanium oxide-based catalyst by gelling a hydrolyzed sol of a titanium alkoxide and a silicon alkoxide with a molar ratio of (1−x)TiO2.xSiO2 (x=0-0.5) and calcining the resulting gel at 350-1200° C. The ratio of silica added to titanium oxide is low in this titanium oxide-based catalyst and, in one example, the molar ratio of titanium oxide to silica (TiO2:SiO2) is 0.95:0.05 and the titanium oxide catalyst prepared by 2-hour calcination at 500° C. shows a specific surface area of 160 m2/g.
According to a method proposed in JP2000-254,493 A, silica-modified titanium oxide for use as a catalyst carrier is prepared by allowing a mixture of a titanium alkoxide and a silicon alkoxide to react in an alcohol solvent and calcining the reaction product. This silica-modified titanium oxide for use as a catalyst carrier has an atomic ratio Ti/Si of 5-50 and shows a BET specific surface area of 90 m2/g or more even when calcined at a temperature as high as 800° C. or above. One example give the results that, when the Ti/Si atomic ratio is 10, silica-modified titanium oxide obtained by calcining at 600° C. shows a specific surface area of 185 m2/g.
A proposal made in JP2000-220,038 A relates to the preparation of titanium oxide fibers containing catalyst components in the following four steps: {circle around (1)} a titanium alkoxide is dissolved in a solvent and allowed to undergo hydrolysis and polymerization reactions by addition of water to produce a polymer; {circle around (2)} the polymer is dissolved in an organic solvent to form a spinning liquid; {circle around (3)} the spinning liquid is spun to form precursor fibers; {circle around (4)} the precursor fibers are treated with steam before and/or during calcination. According to this method, a silicon compound is added in such an amount in the step {circle around (1)} or {circle around (2)} as to adjust the silica content preferably to 5-30 wt % and one example gives the results that the contents of silica and V2O5 are respectively 12 wt % and 19 wt % and the fibrous titanium oxide catalyst obtained after 1-hour calcination in air at 500° C. shows a specific surface area of 173 m2/g.
A method for preparing an alumina-titanium oxide composite catalyst carrier is disclosed in JP5-184,921 A comprises adding a titanium hydroxycarboxylate and/or a sol of titanium oxide and hydroxide and a hydroxycarboxylic acid to oxide and/or hydroxide of aluminum so that the molar ratio of titanium oxide to alumina becomes 2.0 or less and the molar ratio of the hydroxycarboxylic acid to the aforementioned titanium oxide becomes 0.2-2.0 and calcining the kneaded mixture. In one example, a catalyst carrier prepared by calcining at 600° C. for 2 hours and showing a molar TiO2/Al2O3 ratio of 1.53 and a molar hydroxycarboxylic acid/TiO2 ratio of 1.0 shows a specific surface area of 200 m2/g.
A method proposed in JP08-057,322 A for preparing a phosphorus-containing titanium oxide catalyst carrier comprises hydrolyzing a titanium salt to give a hydrated cake of titanium oxide, adding a specified amount of phosphorus to the cake, plasticizing the mixture and molding and calcining in accordance with a specified procedure to give a titanium oxide catalyst carrier containing 1-5 wt % of phosphorus computed as oxide. In an example, a titanium oxide catalyst carrier obtained by 2-hour calcination at 500° C. shows a specific surface area of 108 m2/g.
A method proposed in JP07-232,075 A for preparing a catalyst for removal of nitrogen oxide comprises mixing an oxide or hydrated oxide of titanium and phosphorus so that phosphorus accounts for 0.1-6 wt % of titanium oxide in the mixture, calcining the mixture at 450-800° C. and applying vanadium to the calcined product. In an example, titanium oxide prepared by 2-hour calcination at 550° C. and containing 2.5 wt % of phosphorus relative to titanium oxide shows a specific surface area of 125 m2/g before application of vanadium.
Although the aforementioned technique involving addition of a secondary component such as silica, alumina and phosphorus to titanium oxide can help to improve heat stability and give porous titanium oxide that is capable of maintaining a large surface area even after a high temperature heat treatment, the technique in question is not capable of controlling the pore size and pore distribution of porous titanium oxide sharply and the scarcity of pore sizes optimal for the reaction in the catalyst has made it difficult to obtain sufficient performance in respect to such factors as reaction selectivity, activity and catalyst life.
It is important that catalyst carriers and catalysts used in a variety of chemical reactions have not only a large specific surface area and good heat stability but also a precisely controlled pore structure in respect to pore size and pore distribution. Generally, it is important that molecules taking part in a chemical reaction diffuse readily to the active sites of a catalyst to achieve good contact and readily come off the active sites upon completion of the reaction. For this purpose, it is important that the pore size is controlled to suit the target reactants. That is to say, it is important that catalysts offer no resistance to the diffusion of the reactants and, in addition, they are devoid of too small pores which are not effective for the reaction or too large pores which are wasteful. An ideal catalyst is the one that has pores controlled to fit the aim of the reaction. For example, the effective pore diameter of a catalyst varies from reaction to reaction and it is 6-10 nm for hydrodesulfurization of gas oil, 8-15 nm for hydrodesulfurization of heavy oil, 15-30 nm for hydrodemetallization and 20-40 nm for asphaltene removal.
From the aforementioned point of view, attempts such as the following have been made to prepare porous titanium oxide having a controlled pore structure in respect to pore diameter, pore distribution and the like.
A method proposed in JP60-50,721 B for preparing porous inorganic oxides comprises a step in which a hydrosol to serve as a seed is obtained, a step in which the pH of the hydrosol is swung between the hydrosol-dissolving range and the hydrosol-precipitating range (pH swing operation) thereby causing growth of crystals and coarsely agglomerating the hydrosol and a step in which the coarse agglomerate of the hydrosol is dried and calcined to give a metal oxide. This method can surely yield titanium oxide with a sharply controlled pore distribution; however, it was difficult by this method alone to prepare a titanium oxide catalyst which shows no decrease in specific surface area nor loss of activity as a result of heat history involving heat applied to calcination in the catalyst preparation and reaction heat evolved in the reaction system.
A method proposed in JP06-340,421 A for preparing porous titanium oxide consists of adding ammonia water to a hydrolyzable titanium compound, for example, titanium tetrachloride, to form hydrated titanium oxide, adding a polybasic carboxylic acid to the hydrated titanium oxide to form a chelate, shifting the pH from acidic to neutral by an alkali to separate an organic titanium oxide compound, deflocculating the resulting organic titanium oxide compound with an inorganic acid and further calcining the deflocculated product to give porous titanium oxide. There is a description in an example that porous titanium oxide obtained by 24-hour calcination at 300° C. shows a total pore volume of 0.348 ml/g, a BET specific surface area of 112 m2/g and a pore radius in the range of 20-500 Å, mainly in the range of 32-120 Å with the main peak appearing at 120 Å, and it has a larger surface area and a less broad pore distribution than commercially available titanium oxide. However, this method has limited the calcining temperature to a low level of 300° C. in order to maintain a high specific surface area and, besides, the main pores exist in a broad range of 32-120 Å as expressed in pore radius.
A method proposed in JP11-322,338 A for preparing porous titanium oxide with a well-controlled fine structure consists of preparing a titanium-metal composite compound by adding to a solution of a titanium alkoxide in a watermiscible organic solvent one kind or two kinds or more of salts formed by neutralization of a weak acid with a weak base, a weak acid with a strong base or a weak base with a strong acid, water, and one kind or two kinds or more of salts of rare earth metals, and then removing the metal from the composite compound by an acid treatment, if necessary in the presence of a hydrolysis inhibitor. There is a description in an example that porous titanium oxide obtained after 2-hour calcination at 600° C. shows a specific surface area of 90 m2/g or more but a broad pore distribution in the range of 100-600 Å.
Moreover, in the case where porous titanium oxide is used as a catalyst, it is necessary for titanium oxide to have high purity for full manifestation of its catalytic properties.
According to Togari et al. [Togari, O., Ono, T. and Nakamura, M; Sekiyu Gakkaishi 22, (6), 336 (1979)], a catalyst carrier that is a composite compound of Al2O3.TiO2 or SiO2.TiO2 shows an increasingly greater acid strength as the content of Al2O3 or SiO2 increases. Further, as described in JP08-57,322 A, strong acid sites develop as the content of phosphorus in titanium oxide increases. Consequently, strong acid sites on the catalyst facilitate formation of coke and deactivation of the catalyst in the hydrodesulfurization of petroleum fractions and, according to the studies conducted by the inventors of this invention, the purity of titanium oxide on the oxide (TiO2) basis is 97 wt % or more, preferably 98 wt % or more, in order to maintain a high desulfurization activity per unit specific surface area which is characteristic of titanium oxide and to suppress formation of coke.
By the way, sulfur components and nitrogen components contained in hydrocarbon oils derived from petroleum or coal are converted to sulfur oxides and nitrogen oxides when the hydrocarbon oils are burned as fuel. They cause air pollution when discharged into air or they act as catalyst poisons when formed in the decomposition or conversion reactions of the hydrocarbon oils thereby lowering the efficiency of these reactions. Furthermore, sulfur components in fuel oils for transportation are also poisons for those catalysts which treat exhaust gases discharged from gasoline- and diesel-driven vehicles.
Under the circumstances, hydroprocessing has been practiced to remove sulfur components and nitrogen components from hydrocarbon oils and a large number of hydrotreating catalysts useful for hydroprocessing have been proposed, for example, such catalysts consist of metals possessing catalytic activity for hydrotreating such as molybdenum (Mo), tungsten (W), cobalt (Co) and nickel (Ni) and catalyst carriers such as alumina, zeolite-alumina, alumina-titanium oxide and phosphorus-silica-alumina as disclosed in JP6-106,061 A, JP9-155,197 A, JP9-164,334 A, JP2000-79,343 A, JP2000-93,804A, JP2000-117,111 A, JP2000-135,437 A and JP2001-62,304A.
Generally, a catalyst consisting of molybdenum and cobalt supported on a catalyst carrier is mainly used where the removal of sulfur components (desulfurization) from hydrocarbon oils is the major objective while a catalyst consisting of molybdenum or tungsten and nickel supported on a catalyst carrier is mainly used where the desulfurization and additionally the removal of nitrogen (denitrogenation) are the objectives. The use of nickel here is said to be due to its high capability of hydrogenating aromatic compounds.
The greater part of nitrogen components in hydrocarbon oils occurs as aromatic compounds and, when the removal of the nitrogen-containing aromatic compounds is effected by hydroprocessing, the hydrogenation of the aromatic rings takes place and the rupture of C—N bonds ensues. Thus, the denitrogenation reaction progresses via elimination of nitrogen as ammonia. For this reason, an enhanced capability of hydrogenating aromatic compounds is required in the denitrogenation reaction. As a results, there rises a problem of an increased consumption of hydrogen when hydrocarbon oils are hydrorefined in the presence of a hydrotreating catalyst containing nickel.
The 4th Report of the Central Environmental Council of the Ministry of the Environment, Japan, entitled “What the countermeasures should be for reduction of automobile exhaust gases in the future” presented in November, 2000 states that it is appropriate to reduce the sulfur components in gas oil or fuel oil for diesel engines from the current level of 500 ppm to 50 ppm by the fiscal year 2004 and a still further reduction of sulfur components is desirable in the future. As for the nitrogen components in hydrocarbon oils such as gas oil, they not only deteriorate the quality of product oil by coloration but also poison and deteriorate hydrotreating catalysts during hydroprocessing and they should desirably be removed as much as possible.
However, hydroprocessing by the use of the aforementioned conventional hydrotreating catalysts does not necessarily give sufficient performance in desulfurization and denitrogenation and it becomes necessary to conduct hydroprocessing under severer conditions in order to reduce the sulfur components in gas oil to 50 ppm or less. For example, it would be necessary to reduce the throughput to ⅓ or to roughly treble the amount of the catalyst. Reduced throughput would call for a critical review of the production schedule of an oil refinery while an increase in the amount of catalyst would require additional installation of, say, two reactors. Or, it would be necessary to raise the reaction temperature by 20° C. or more and this would be done at a great sacrifice of the catalyst life. These measures would forcibly incur a great deal of economic burden. Moreover, it is difficult to remove nitrogen components by hydroprocessing to the same extent as in the case of sulfur components and any attempt to effect hydroprocessing to remove nitrogen components at a high rate would require excessive consumption of hydrogen and this would necessitate installation of a new apparatus for producing hydrogen at an oil refinery where excess hydrogen is barely available.
As described above, it is not possible to prepare a desulfurization catalyst of high activity and this is for the following reason: in a hydrotreating catalyst consisting of the principal catalyst component molybdenum and the promoter component cobalt and a catalyst carrier mainly composed of alumina, the amount of molybdenum is normally 25 wt % or less on the oxide basis and any attempt to increase molybdenum any further would cause agglomeration of molybdenum on the catalyst carrier and this would prevent molybdenum from undergoing high dispersion and effectively manifesting catalytic performance and would additionally produce such adverse effects as blocking pores and decreasing surface area and pore volume thereby failing to exhibit a needed activity.